In order to detect an amount of movement of the image between frames, it is ideal to calculate how far and which direction each pixel in the image moved and this method ensures the best precision in vector detection. However, this method requires large-scale hardware and takes a lot of time, so that it is difficult to realize the above method. Thus, there is a general method for determining the moving vector in the whole screen by finding an amount of movement of several pixels (referred to as representative points hereinafter) in the screen.
FIG. 20 is a diagram showing a general representative point calculating circuit. FIG. 21 is a diagram showing the relation between block and the representative point in the image according to the conventional example of FIG. 20. The image of one field is divided into the prescribed number of blocks 901 and one representative point R.sub.ij 902 is provided in the center of each block. The level difference between the representative point in the previous frame and the whole pixels P.sub.ij (x,y) 903 in the block is calculated in each block.
In FIG. 20, an input video signal (a) is A/D converted by the A/D converter 1. Then, the video signal converted to a digital signal by the A/D converter 1 is calculated as follows. A calculation according to the pixel in the block 901 will be described as an example. A prescribed pixel to become the representative point 902 in the block 901 is written in a prescribed region of a representative point memory 802 through a latch circuit 801. Data stored in the representative point memory 802 is read one frame later and applied to a difference absolute value calculating circuit 805 through a latch circuit 803. On the other hand, the other A/D converted data of the video signal is applied to the difference absolute value calculating circuit 805 through a latch circuit 804. The absolute value of the difference between the representative point signal (f) of the previous frame output from the latch circuit 803 and the pixel signal (g) of the present frame output from the latch circuit 804 is calculated by the difference absolute value calculating circuit 805. These calculations are per-formed in each block and an output signal (h) from the difference absolute value calculating circuit 805 is added one after another to tables corresponding to the same address of the pixel in each block of the accumulated addition table 806. The result of the addition in the tables is input to a table value comparator 807 and finally it is determined how far and which direction the image position moved for one frame, that is, a moving vector value (i) is determined from a block address in which the result of the addition is the minimum.
More specifically, the absolute value of the difference between the representative point R.sub.ij and a signal S.sub.ij (x,y) positioned in a horizontal direction x and a vertical direction y is found and then it is added to x and y of the same position to each representative point and then it is set as an accumulated addition table D.sub.xy, D.sub.xy is represented as follows; EQU D.sub.xy =.SIGMA..vertline.R.sub.ij -S.sub.ij (x,y).vertline.
Then, the minimum x and y in the D.sub.xy are set as moving vectors in the horizontal direction and in the vertical direction, respectively.
In the above structure, however, since the moving vector is found in a plane manner, i.e., two dimensionally, the accumulated addition tables whose number corresponds to the number of whole pixels in the block are required. For example, if the number of the pixels are 32 in the horizontal direction and 16 in the vertical direction, 512 (=32.times.16) accumulated addition tables are required, with the result that the scale of the circuit is increased. In addition, when the moving vector is calculated, all the data in the accumulated addition tables is compared, with the result that the number of comparisons is very large and also it takes a lot of time.
Meanwhile, inventors of the present invention propose an improved moving vector detecting apparatus in Japanese Patent Published Application No. 1-277539, in which two intersecting one-dimensional moving vectors are obtained from an accumulated addition table corresponding to a pixel on a vertical straight line passing through a representative point and an accumulated addition table corresponding to a pixel on a horizontal straight line passing through the representative point and a two-dimensional moving vector is calculated from these two intersecting one-dimensional moving vectors.
FIG. 22 is a block diagram showing the moving vector detecting apparatus for determining a vector on the screen by detecting two intersecting one-dimensional moving vectors, disclosed in the Japanese Patent Published Application No. 1-277539. In FIG. 22, an input video signal (a) is A/D converted by an A/D converter 1 and then a prescribed pixel in the block 2020 (shown in FIG. 23) to become the representative point 2021 is written in a representative point memory 813 through a latch circuit 812. The data stored in the representative point memory 813 is read one frame after and then applied to a vertical absolute value circuit 819 and a horizontal absolute value circuit 820 through a latch circuit 814. On the other hand, the data of the A/D converted video signal is applied to the the vertical absolute value circuit 819 through a vertical pixel latch circuit 816 which latches at a timing corresponding to the pixel in the vertical direction of the representative point and also applied to the horizontal absolute value circuit 820 through a horizontal pixel latch circuit 818 which latches at a timing corresponding to the pixel in the horizontal direction of the representative point. A representative point signal (c) in the previous frame which is output from the latch circuit 814 and a pixel signal (d) in the present frame which is output from the vertical pixel latch circuit 816 are calculated at the vertical absolute value circuit 819 to find the absolute value of the difference between them. The representative point signal (c) in the previous frame which is output from the latch circuit 814 and a pixel signal (e) in the present frame which is output from the horizontal pixel latch circuit 818 are calculated at the horizontal absolute value circuit 820 to find the absolute value of the difference between them. These calculations are performed every block and an output signal (f) from the vertical absolute value circuit 819 is added one after another to tables corresponding to the same address of the pixel in each block in the vertical accumulated addition table 821 and an output signal (g) from the horizontal absolute value circuit 820 is added one after another to tables corresponding to the same address of the pixel in each block in the horizontal accumulated addition table 822. The result of the horizontal accumulated addition table 822 and the result of the vertical accumulated addition table 821 are input to the one-dimensional vector detecting means 823 and then a vertical moving vector (h) and a horizontal moving vector (i) are detected therein. The vertical moving vector (h) and the horizontal moving vector (i) thus obtained, which are two intersecting one-dimensional vectors, are input to two-dimensional vector calculating means 824 and then it is found how far and in which direction the image position moved for one frame at the final stage, i.e., a two-dimensional moving vector value (j) is determined.
FIG. 23 is a view showing the relation between the block and the representative point of the image according to FIG. 22. The image of one field is divided into prescribed number of blocks 2020 and one representative point R.sub.ij 2021 is provided at the center of each block. The level difference between the representative point in the previous frame and a pixel S.sub.ij (0,y) 2070 in the vertical direction of the representative point and a pixel S.sub.ij (x,0) 2071 in the horizontal direction of the representative point in the block is calculated every block and then the level difference is added one after another to tables corresponding to the same address of the pixel in each block. It is found that how far and which direction the image position moved in the vertical and horizontal directions for one frame by a block address in which the addition result is the minimum in each area, i.e., the vertical moving vector value (h) and the horizontal moving vector value (i) are determined.
More specifically, a vertical accumulated addition table D.sub.y is obtained by finding the absolute value of the difference between the representative point R.sub.ij and a signal S.sub.ij (0,y) positioned apart from that in the vertical direction by y and adding the same over those at the same position y relation with respect to each representative point, and a horizontal accumulated addition table D.sub.X is obtained by finding the absolute value of the difference between the representative point R.sub.ij and a signal S.sub.ij (x,0) positioned apart from that in the horizontal direction by x and adding the same over those at the same position x relation with respect to each representative point. At this time, they are represented by the following equations, that is; EQU D.sub.X =.SIGMA..vertline.R.sub.ij -S.sub.ij (x,0).vertline. EQU D.sub.Y =.SIGMA..vertline.R.sub.ij -S.sub.ij (0,y).vertline.
Then, the minimum x and y in the D.sub.x and D.sub.y are set as the horizontal moving vector (i) and the vertical moving vector (h), respectively.
A description is given of the detecting precision in detecting two intersecting one-dimensional moving vectors from the vertical accumulated addition table and the horizontal accumulated addition table.
Generally, an auto-correlation function of an image has a statistical characteristic to be approximated by Laplace distribution (negative exponential function). However, for simplification, it is supposed that the correlation decreases in proportion to the distance, both in the horizontal and vertical directions, in the following description.
FIGS. 24(a)-(c) show a case where the image is at a standstill and FIGS. 25(a)-(c) show a case where the image moves in a diagonal direction. FIGS. 24(a) and 25(a) show the accumulated addition table corresponding to the whole number of pixels in the block in which a horizontal direction (x), a vertical direction (y) and a value of the accumulated addition table (z) are represented by three dimensions with the representative point as the origin. In addition, FIGS. 24(b) and 25(b) show the vertical accumulated addition table D.sub.Y obtained by finding the absolute value of the difference between the representative point R.sub.ij and a signal S.sub.ij (0,y) positioned in the vertical direction y thereof and adding the same over those at the same position y relation with respect to each representative point, in which the vertical direction (y) and the value of the accumulated addition table (z) are represented by two dimensions with the representative point as the origin. FIGS. 24(c) and 25(c) show the horizontal accumulated addition table D.sub.X obtained by finding the absolute value of the difference between the representative point R.sub.ij and a signal S.sub.ij (x,0) positioned in the vertical direction x thereof and adding the same over those at the same position x relation with respect to each representative point, in which the horizontal direction (x) and the value of the accumulated addition table (z) are represented by two dimensions with the representative point as the origin.
When the image is at a standstill, the accumulated addition table value is in the form of an inverted cone with the origin (0,0,0) as an apex as shown in FIG. 24(a). At this time, the value of the vertical accumulated addition table shown in FIG. 24(b) is the minimum when y=0 and the horizontal accumulated addition table shown in FIG. 24(c) is the minimum when x=0. Therefore, it is found that the vertical moving vector and the horizontal moving vectors are both 0 vector.
If the picture moves largely by x.sub.1 in the horizontal direction and slightly by y.sub.1 in the vertical direction for one frame, the accumulated addition table is in the form of the inverted cone with (x.sub.1,y.sub.1,0) as an apex as shown in FIG. 25(a). At this time, the vertical accumulated addition table is the value of a section of the cone at the plane of x=0 as shown in FIG. 25(b) and the horizontal accumulated addition table is the value of a section of the cone at the plane of y=0 as shown in FIG. 25(c). Then, as can be seen from FIG. 25(b), the value of the vertical accumulated addition table is the minimum when y=y.sub.1. Also, as can be seen from FIG. 25(c), the value of the horizontal accumulated addition table is the minimum when x=x.sub.1. Accordingly, it is found that the vertical moving vector (h) is (0,y.sub.1) and the horizontal moving vector (i) is (x.sub.1,0). However, because of the difference in amount of movement between the horizontal direction and the vertical direction, a steep hyperbolic curve is shown in FIG. 25(c) while a gentle hyperbolic curve is shown in FIG. 25(b).
In case of an actual horizontal accumulated addition table, an error of .DELTA.z in the direction of z exists because of a quantization error, an round-off error, a difference in horizontal correlation level of the image, a local movement or the like. Therefore, although detecting precision is high in the case of the steep hyperbolic curve shown in FIG. 25(c), it is low in the case of the gentle hyperbolic curve shown in FIG. 25(b).
As described above, when the moving vector is found in a plane manner, i.e., two-dimensionally, the number of the accumulated addition tables increases, with the result that the scale of the circuit is increased and the calculation takes a lot of time. In addition, in the case where two intersecting one-dimensional moving vectors are detected from the accumulated addition table corresponding to the pixel on a vertical straight line passing through the representative point and the accumulated addition table corresponding to the pixel on a horizontal straight line passing through the representative point, if the vector moves largely toward either axis, the moving vector detecting precision at the other axis is deteriorated.